Wireless communications devices such as the BlackBerry™ by Research in Motion Limited enable users to download map content from web-based data sources such as BlackBerry Maps™, Google Maps™ or Mapquest™. Downloaded map content is displayed on a small LCD display screen of the wireless communications device for viewing by the user. The user can pan up and down and side to side as well as zoom in or out. Due to the small display on the device and due to the limited over-the-air (OTA) bandwidth, there is a need to optimize the delivery and handling of the map data.
With the increasing availability of wireless communications devices having onboard Global Positioning System (GPS) receivers for providing location-based services (LBS), the efficient delivery and handling of map data is increasingly important.
When the user requests a route between starting and destination locations (that is, the end-points of the route), the server generates a path represented by a series of waypoints between the two user-defined end-points, as well as a set of directions (for example “driving instructions”) for following the route. Typically, the directions are associated with “decision points” that are located along the route, for example at each intersection or junction where the user must make a turn or take one of two or more possible roads.
For the purposes of understanding the present description, the “route” refers to a sequence of roads, highways, etc. that the user must follow in order to travel between the starting and destination points. The route also provides a context for directions intended to guide the user along the route, and decision points. A “path” is an abstract representation of the route, which is intended to be graphically superimposed over a map display of all or part of the route, so as to assist the user in identifying and following the route. Typically, the path is represented by a set of waypoints distributed along the route. Representation of the path as a set of waypoints enables simple computation and storage of the path, and allows the path to be overlayed onto the map display with proper scaling. Normally, the set of waypoints defining the path will be downloaded to the wireless communications device as part of the data file containing the route information. In order to reduce demand for processor resources on the wireless communications device, the path is normally displayed as a series of straight line segments (otherwise known as vectors) extending between successive waypoints. The waypoints themselves will not normally be shown on the display.
A limitation of the conventional methods is that if the path is very long, the number of waypoints may become too large to send over-the-air in a timely fashion. Therefore, if the path contains more than a maximum number of waypoints which can reasonably be transmitted over-the-air in a timely fashion, some waypoints must be removed to reduce the total number of waypoints. A disadvantage of this approach is that reducing the number of waypoints also has the effect of reducing the accuracy with which the path traces the route on a map display. While the waypoints may be accurately “pinned” to the actual route shown on the map display, the straight line segments of the path display will tend to deviate from the route, and this deviation will tend to increase as the curvature of the route (roads) and/or the separation between the remaining waypoints increases.
Many generalization algorithms are known which are intended to eliminate waypoints while minimizing an overall deviation of the path from the desired route. However, a route through an area with many roads (such as within a city, or at a highway interchange, for example) presents a unique problem, since it is desired that the path follow the actual roads traversed by the route as exactly as possible, even with a reduced set of waypoints. An algorithm that minimizes the overall error of the path, on the other hand, may produce a path that lies close to a road traversed by the route, but not actually on it, and this situation may persist for much of the route between the starting point and the destination point.
FIGS. 1a-b, are maps generated by a well-known mapping service, which illustrate this problem. FIG. 1a shows a portion of a route 2 in which the user is directed to exit “Moreno Valley Freeway” 60 and merge onto “Riverside Freeway” 91. The actual route 2 is shown in dashed line. However, as can be seen in FIG. 1a, the path 4, shown in solid line, deviates markedly from the route 2, due to the reduced set of waypoints used to define the path 4. FIG. 1b shows a portion of a route 2 within a city, in which the user is directed from 1st avenue to the Cabrillo Freeway 163 via a series of access lanes and on-ramps. As in FIG. 1a, the actual route 2 is shown in dashed line. The path 4, shown in solid line, deviates markedly from the route, due to the reduced set of waypoints. In this case, the deviation is so severe that the path display provides little useful guidance to the user, even though the overall error of the path—measured along the entire length of the route—has been minimized.
FIG. 2 illustrates another example, in which a pair of decision points 19 and 20 are shown along a route in which the user is guided onto “Berliner Ring” via an on-ramp. However, the path fails to follow the on-ramp, and even misses one of the decision points (19, 20) altogether, due to the reduced set of waypoints.
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.